hi sanderbreur,
If you want to have SnapCenter Repository (NSM)database on AG, you should install on primary host of AG by giving Hostname\Instance. Once the installtion of SnapCenter is done. you need to migrate the NSM to Netapp lun. using protect-smrepository. Once NSM is on NetApp lun manually create NSM database on AG.
you need to change the web.config file using C:\inetpub\wwwroot\SnapCenter\Web.config
change <DBServer> value= "AG lister name and port"
example:
<add key = "DBServer" value="ag-list-nam, 5010"/>
once done same the changes and restart SMCORE "SnapCenter SMCore Service" server in SnapCenter.
Please refer documentation of SnapCenter Plugin for MSSQL under session:
"Prerequisites for protecting snapcenter repository".
if you need help please reach out to netapp support they should able to help you.
Thanks
Neeraj